The invention relates in general to the use of terminal location information in mobile telephone systems based on cellular networks and in particular to determining and indicating to the telephone user the call tariff applicable on the basis of the location of the terminal.
Digital mobile telephones operating in cellular networks are quickly becoming the most important personal communication media. Communication in a cellular network occurs via fixed base stations, and mobile telephone users are free to move with their phones within the coverage area of the system consisting of base station coverage areas, or cells. In addition to mobile phones, many kinds of other terminals may also be used in cellular networks, but below we will use the mobile telephone as an example.
Pricing of calls is based on an agreement between a user and network operator. From the point of view of competition between operators it is preferable that an operator be able to offer different pricing systems amongst which a suitable alternative can be selected for each user. One known pricing system is the so-called home area pricing, in which a user (or rather his or her terminal or the module intended for identifying the user) is assigned a so-called home area comprising the coverage areas of a certain base station or certain base stations and in which the user can make and receive calls at a cheaper rate. Then it is needed a method to determine whether or not the user is located in his or her home area. Usually the user also wants that his or her telephone somehow indicates whether or not the home area tariff is being applied, and preferably the telephone should make this indication while in the idle mode, ie. when no telephone connection exists at the telephone.
It is known several methods to implement said home area detection and tariff indication. In one system, a telephone listens to the identification code, or CellId parameter, of the base station in the coverage area of which it is located and compares that code with the list of home area base stations, stored in the subscriber identity module (SIM) of the telephone (terms related to the GSM system can here and below be generalized to relate to other systems as well, where applicable). In a second system, an alphanumeric code is transmitted via base stations in the so-called cell broadcast format, and that code may be, for example, the name and/or postal code of a town or city which will be provided directly on the display of a telephone, whereby the user shall determine whether or not he or she is in the home area or not. A third system is also based on said cell broadcast, but there each base station sends its geographical coordinates to the telephone. The home coordinates of the telephone and the radius of the home area are stored in the telephone, preferably in its SIM. The data processing unit of the telephone calculates the geometrical distance between the base station coordinates and the coordinates stored in the SIM, and if the distance is smaller than said home area radius, the telephone concludes that it is in the home area.
There are some problems involved in the prior art systems described above. In the first system, in which the identification codes of the home area base stations are stored in the telephone memory, the list has to be revised every time that a new base station is installed in the home area or the operation of the base stations is otherwise changed. The wider the home area, the more frequent the need for revision, because in a wide area there will be changes more frequently than in a small area. From the point of view of the network operator the updating work gets more laborious as the number of home area users in an area gets bigger. The memory capacity of the SIM is also limited and in an area with heavy traffic there are so many base stations that the home area base station list easily becomes very long. A CellId parameter of a base station cannot be used in other base stations to enlarge the home area of a user since then also the home areas of all the other users registered in the same home area would be enlarged even if their agreements only covered the original home area. The second system primarily puts a strain on the user's memory, because a wide home area may mean many alphanumeric home area codes to memorize. In addition, if the user is assigned several zones with different tariffs, it easily becomes an overwhelming task to remember all the relevant details. In the third system, the home area always constitutes a circle around the home point, which is not practical in all cases.